Modulation system



Patented 5, 1942 MODULATION SYSTEM William W. Hansen and Russell H. Varian, Stanford University, Calif., assignors to The Board of Trustees of The Leland Stanford, Junior, University, Stanford University, Calif., a corporation of California Original application April 14, 1938, Serial No. 201,898. Divided and this application March 27, 1940, Serial No. 326,150. In Great Britain March 17, 1939 20 Claims.

This invention relates, generally, to radio transmission and reception, and the invention has reference more particularly to radio transmitting and receiving apparatus operating at ultra high frequencies, and employing 'cavity resonator devices, or tubes, excited by electron streams.

The present application is a division of copending application Serial Number 201,898, filed April 14, 1938, for Radio transmission and reception.

The principal object of the present invention is to provide, in connection with ran electron stream excited cavity resonator or tube oscillator or amplifier, means for effecting modulation of the output of such oscillator and/or amplifier.

Another object of the present invention is to provide a novel oscillator and/or amplifier of the above character employing electrode means and a connected modulating supply for selectively effecting frequency, phase and amplitude modulation at will of the output of said oscillator and/or amplifier.

Still another object of the present invention is to provide, in connection with an electron stream type oscillator and/or amplifier, electrode means excited at the frequency of the apparatus for effecting optimum operating efficiency of the apparatus.

A further object of the invention is to provide electrode tube means for effecting frequency modulation of an electron stream type oscillator and/or amplifier, said apparatus employing additional electrode means for eliminating amplitude modulation of the output of the apparatus.

Still another object of the invention is to provide feed back means in an oscillator, which feed back means employs a cavity resonator having a high Q for effecting frequency stability of the oscillator.

Other objects and advantages will become apparent from the specification, taken in connection with the accompanying drawing wherein the invention is embodied in concrete form.

. The single figure of the drawing is a circuit diagram of a typical transmitter conforming to the principles of the present invention. Referring to the drawing, there is disclosed an electron stream excited cavity oscillator consisting of cavity, or hollow resonators 4 and 5 that are excited by a stream of electrons emitted from the cathode l, and accelerated by accelerating voltage supplied by battery 3, connected to the cathode and to the resonator 4 having the accelerating grid 2. The cavity resonators 4 and 55 5 in operation contain standing electromagnetic waves having a'frequency determined bythe dimensions of the cavities. The electrons are projected as a stream through the grid 2 and through the grids of successive resonators 4 and 5. Resonators 4 and 5 are back-coupled through use of loops 8, l0 and 9, 1 and a large resonator 6,

Elements 4 and 5 operate as an oscillator largely as described in the co-pending application Serial No. 168,355, but instead of coupling directly from the second device 5 to device 4, as described before, .they are coupled through a third hollow resonator device 6 preferably made considerably larger than 4 or 5 so that it will have a fundamental frequency much lower than the fundamental frequency of devices 4 and 5, and operating at one of its harmonic frequencies. As one of the consequencesof the theory of hollow resonator operation set forth in application' Serial No. 92,787 it is well known that the ratio of the inductive reactance to the resistance of a resonant device is higher for the harmonics than for the fundamental frequency. Also, from the theory of radio circuits it is known that frequency stability is a function of this ratio which is commonly designated by the letter Q. Accordingly in this invention device 5 is operated on one of its harmonics for coupling between devices 4 and 5. A value of Q of the order of 104 is not uncommon in these devices operating on the fundamental frequencies.v It can be increased to or ll)8 by using one of the higher harmonies. This, of course, increases the frequency stability of the system.

The theory developed for the velocity grouped electronic oscillator set forth in application No. 168,355 shows that the bunched beam contains many harmonics of the fundamental bunching frequency and that the beam can excite circuits oscillating at the harmonic frequencies as Well as at the fundamental. This property is used as shown in the figure wherein the bunched electron beam after emerging from device 5 is used to excite another device Il whose fundamental frequency is that of a Fourier harmonic of the frequency of the devices 5 and 4. Devices 4 and 5 act as a master oscillator and clevice Il as a frequency multiplier. The output of device Il is shown amplified in devices l2 and i3 which are provided with an electron beam emitter I4 and the usual battery and collimating grid. The amplified high frequency energy is shown as connected for delivery by a coupling loop l5 to an antenna I6 from which it is radiated. Obviously energy could be radlated directly from device II, if desired.

Modulation of the signal can be accomplished in a number of ways as indicated in the figure. One way is to use a tube I'I between any two of the resonant devices, or cavity resonators, and to vary its potential as by using a transformer I8 connected to a modulating device such as a telephone transmitter I9, or some other means of producing a modulating voltage.. The tube II is illustrated in three different possible locations in the figure. 'I'he eiect of the modulating tube II is different in the several locations shown. In its location between cavity resonators 4 and 5 it shifts the phase of energy transfer between resonators 4 and 5. In such a system where feed backoccurs, as it does in this instance, shifting the phase between two resonant components of the circuit changes the frequency somewhat as a function of the voltage applied and may also be made to change the amplitude of the oscillations.

In the arrangement shown "pure frequency modulation can be attained by having the voltage on tube I1 control also a volume control 20, as of the amplifier or potentiometer types, connected to a modulating grid 2| which counteracts the amplitude modulating effect of tube I'I. By this means tube I1 can modulate the frequency of the system without modulating it amplitude. i

The effect of modulating tube II in the locations between cavity resonators and II, or between resonators I2 and I3 is to phase modulate the output of the system, the phase shift of the oscillationsin member II or I3, as the case may be, being a function of the voltage applied to tube I1. In phase modulation the oscillator including cavity resonators 4, 5 and 6 oscillate steadily at their normal stable frequency, while the frequency multiplying resonator II and the amplifying resonators I2 and I3 oscillate at the same frequency but with shifting phase between resonator 5 and the output resonator I3. The phase shifting effect between resonators 5 and II is the same as between resonators 4 and 5, but there is no feed back from resonator II to change the system `frequency. If cavity resonator II is operating at a frequency that is the nth harmonic, where n is an integer, of the frequency of cavity resonators 4 and 5, the phase shift of the nth harmonic component of bunching in the electron beam in tube I1 is n times the phase shift of the fundamental component. The effect of the tube I'I between cavity resonators I2 and' I3 is the same as when it is between cavity resonators 5 and II as far as output at I6 is concerned. The entire effect is phase modulation. f

Amplitude modulation of the system can be accomplished by the grid 2I, as described in Serial No. 185,382. It can also be accomplished by spaced plates 22 in the figure connected respectively to the sides of the secondary of transformer I8 which is supplied with a suitable modulating voltage. Plates 22 deect the electron stream passing between them so that the effectiveness of the stream is varied as a function of the` stream deflection by the plates 22. The effectiveness of the stream is a maximum when it is centered in the system and such effectiveness is reduced by deflection to either side. Accordingly, the system is adjusted so that without modulation the beam rests at some interaasnoss mediate position where the effect is less than maximum so that double frequency modulation is avoided. The same effect can be attained by blocking olf a portion of the grids of cavity resonator 5.

When the tube I1 is located between cavity resonators I2 and I3 the amount of phase shift per unit of applied voltage is somewhat different than that obtained when this tube is placed 'oetween resonators 5 and II.

Plates 22 have another possible use. If, in-

stead of exciting them at a modulating frequency.

they are excited at the oscillation frequency of the system the electron beam between the plates 22'can be shifted at the oscillating frequency. By adjusting the assemblage so that the electron stream is shifted considerably to one side reaching its maximum shift when the electron stream bunches at the grids of cavity resonator 5 are in the phase least productive of system oscillation, and so that the stream is centered when the stream bunching at resonator 5 is at its optimum phase for producing oscillation, the bunching economy of the stream is improved. That is, the possible transfer of stream energy to the oscillating device 5 may be increased, by deflection, somewhat beyond the theoretical maximum percentage possible with bunching alone. For most effective operation the plates 22 would be replaced by a defiecting cavity resonator, as shown in Serial No. 193,268.

Amplitude modulation can be accomplished by the additional method of changing the voltage between the amplifier electron emitter I4 and cavity resonator I2 as shown in the figure by use of modulating transformer I8. An additional method of accomplishing phase modulation is to change the voltage between amplifier resonators I2 and I3 by use of transformer I8'.

The frequency adjustment of the several cavity resonators referred to in the ligure can be ac-` complished by any of the methods shown in the parent case above referred to as by use of handles 4I for turning elements 36. Usually all such cavity resonators but one in a system should be adjustable. In the gure, the unadjusted device is shown at 5. The frequency adjustment of resonator 6 is accomplished by varying condenser 43 connected to loop 38.

In the co-pending application Serial No. 185,382 transmitters are shown used with parabolic reflectors. Such a combination is generally applicable for transmitters of the frequencies of the order of 109 cycles per second or higher. It is also in frequencies of this order that frequency stability enhanced by use of resonator 5 may be very important.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The method of generating high frequency electromagnetic waves through use of electron stream coupled circuits operating at a fundamental and harmonic frequencies respectively, which consists of producing in an electron stream changes in electron velocity, causing the electron stream to excite a circuit having a selected fundamental frequency for sustaining operation of the system, and causing the same stream thereafter to excite a circuit having a frequency which is a harmonic of the said fundamental frequency.

2. The method of modulating the envelope of the high frequency output of ultra-high frequency electron beam apparatus which consists of initiating recurrent changes in electron density of the beam at a region in the path thereof at ultra-high frequency, and effecting modulation of said ultra-high frequency by changing the time of flight of the electrons in the beam between said region and a point of energy absorption from the beam in accordance with an applied lower frequency modulation.

3. In an oscillator of the character described having means for producing a velocity grouped electron stream, means for supplying a modulating signal, electrode means connected to be activated by said modulating signal and acting on the stream for effecting frequency and some undesired amplitude modulation of th'e stream, and counteracting electrode means controlled from said modulating signal also acting on the stream for canceling the amplitude modulation thereof.

4. In a device of the character described,

y means including` an emitter and electron accelerating voltage supply means for providing an electron stream, a hollow resonator positioned for receiving the stream, means for exciting said resonator to effect velocity grouping of the stream, and means for varying the electron accelerating voltage to effect amplitude modulation of said stream.

5. In an electron stream excited oscillator having electron grouping and energy interchanging means, feed back means comprising a cavity resonator connected to both said electron grouping and energy interchanging means, said cavity resonator having a high Q for effecting frequency stability of the oscillator.

6. In an electron stream excited oscillator,

A means for producing an electron stream, a pair of spaced hollow resonators excited by the electron stream and a feed back connection between said resonators comprising a hollow resonator operating on a harmonic of its fundamental frequency.

7. In apparatus of the character described, a pair of spaced resonators, means for producing an electron stream for passing through said resonators, and means for applying a modulating voltage to said stream in a region between said resonators to effect phase modulation of the output of said apparatus.

8. In apparatus of the character described, a pair of spaced resonators, means for exciting one of said resonators, means for producing an electron stream and for exciting the other of said resonators by the passage of the stream therethrough, an electrode positioned between said resonators, and means for applying a modulating pbtential to said electrode for modulating the output of said apparatus.

9. In an oscillator of the character described including means for producing an electron stream, means for effecting electron grouping of said stream, spaced means for deriving high frequency electrical energy from said stream, and an additional element for changing the phase of arrival of said electron groups in said energy deriving means to thereby change the frequency of said oscillator.

10. In an oscillator of the character described including means for producing an electron stream, means for effecting electron grouping of 3 quency electrical energy from said stream, back coupling means connected from said energy deriving means to said electron grouping means, and an additional element for changing the A phase of arrival of said electron/groups in said energy deriving means to thereby change the phase of the output of said oscillator.

1l. In apparatus of the character described, means for producing an electron stream, means for initiating electron grouping in said stream at ultra-high frequency, spaced means for deriving oscillatory energy from said grouped stream, and modulating means for altering the time of transit of said groups from said group initiating means to said means for deriving oscillatory energy at a frequency lower than said electron grouping frequency.

12. The method of modulating the output of ultra-high frequency electron beam apparatus which consists of initiating recurrent changes at ultra-high frequency in electron density of the beam at a region in the path thereof, and effecting modulation by thereafter shifting the beam transversely of its course at a frequency lower than said electron density changes at a point between said region and a point of energy absorption from the beam in accordance with the applied modulation.

13. In an electron stream excited oscillator, means for producing an electron stream, a pair of spaced hollow resonators excited by the electron stream, and means for stabilizing said oscillator comprising an additional hollow resonator coupled to at least one of said resonators.

14. In a device of the character described, means including an emitter and electron accelerating voltage supply means for producing an electron stream, hollow resonator means positioned for receiving said stream, and means for varying the electron accelerating voltage for varying the operating frequency of said resonator means.

15. In an electron" stream excited oscillator, means for producing an electron stream, electron grouping and energy interchanging means and a cavity resonator coupled to said electron grouping and energy interchanging means.

16. A harmonic frequency generator comprising, means for producing an electron stream, means coacting with said stream for effecting recurrent grouping of the electrons of the stream at an initial frequency, and hollow resonator means having a natural frequency" that is a multiple of said initial frequency, `said hollow resonator means being positioned to be excited by said grouped stream for supplying a frequency higher than said initial frequency.

17. A high frequency device comprising means for generating a harmonic frequency, said means including means for producing an electron stream, and hollow resonator means positioned for effecting recurrent grouping of the electrons of the stream at a high frequency and for abstracting energy from said stream at a harmonic of said high frequency.

18. The method of shifting the phase of electromagnetic oscillations in an energy absorbing circuit which consists of producing a stream of electrons, passing the stream through an electromagnetic field arranged to change the vector velocity of the electrons, passing the stream of variable vector velocity electrons through the energy absorbing circuit to set up electromagnetic said stream, spaced means for deriving high fre- 76 oscillations therein, and passing the stream through an electric field to change the time interval existing between the act of changing the vector velocity and the resultant delivery of electromagnetic energy to the energy absorbing circuit.

19. The method of electrically adjusting the resonantfrequency of a high frequency electronic device employing an exciting electron stream having a relatively long flight time, said device having electron grouping and energy absorbing means coupled so that the energy absorbing means will excite the electron grouping means, consisting of changing the flight time of electrons oi' the electron stream between the electron grouping and the energy absorbing means to effect a change in the phase of electron groups arriving in the energy absorbing means relative to the oscillations present in the energy grouping means, and vthereby altering the resonant frequency of said device.

20. In a tube structure. avplurality oi mutually spaced resonant circuits arranged to have alternating electric elds, means for passing a stream of electrons through said alternating electric fields, and potential applying means independent of said iirst named means and op'e'rat-l ing on the stream for controlling the iiight time of electrons of the stream between the altemating electric fields oi' said circuits.

WILLIAM W. HANSEN. RUSSELL H. VARIAN. 

